The present invention relates to the area of open communication system testing, and more particularly to a circuit arrangement for processing a communication, i.e. a circuit arrangement with which a communication, which is subdivided into functional layers, is processable by a first layer for a higher layer and/or by a higher layer for the first layer, the first layer being formed by a physical layer.
Open communication systems must be understood as systems that are capable of communicating and cooperating with other systems. Cooperation here means communication between systems for the purpose of completing a joint task or of continually developing a joint initial understanding through the transfer of information.
Manufacturer-independent, flexible communication is accomplished by open systems complying with standardized procedures for information exchange, such as standardized rules and protocols, which determine system behaviour. Such protocols do not only define the internal structure of the open system but also its outward behaviour.
The architecture model for open communication systems was developed by ISO, the International Standardization Organization. The OSI (Open Systems Interconnection) reference model subdivides the necessary functions into a hierarchical layer structure. It has a major influence on the future design of data end systems and data networks. FIG. 1 shows the OSI communication model. Applications 10a, 10b of two end systems 12a, 12b are the real sources and sinks of the communication. One end system 12a, which has one or several applications, 10a communicates with another end system 12b. The communication may take place either directly via a transmission medium 14 or via a transit system 16 which may be a data network or a telephone network.
The OSI reference model is based on the following principles:
Layering of functions required for the communication
Service of the individual layers
Protocols for communication between layers of the same level
The seven layers of the OSI reference model are shown in FIG. 2. The OSI reference model is a thought model in which the communication is subdivided into strictly task-related, functional layers, with layers 1 to 4 forming the so-called transport function and layers 5 to 7 describing application protocols.
In FIG. 2, elements corresponding to those of FIG. 1 have been given the same reference numbers. The figure also shows that the relevant end system is assigned to a use 18a, 18b. 
The following is a description of the individual layers:
Layer 1: Bit transfer layer (physical layer):
                The bit transfer layer provides unsecured transfer of binary signals on a transfer path. It comprises the following functions: parallel/serial conversion, adaptation to the physical properties of the transmission medium, synchronisation, interconnection of transfer sections, status monitoring, activation and deactivation of the transfer path.Layer 2: Securing layer (data link layer):        From the unsecured transfer in layer 1 a secured transfer is achieved with the aid of the functions of layer 2. It includes the following functions: connection/disconnection of a layer 2 connection, transfer control, transfer error monitoring, subdivision of the data of layer 3 into blocks, block numbering, generation and evaluation of test bytes.Layer 3: Switching layer (network layer):        This decides how a network connection between the end systems is set up. Functions: set-up and monitoring of network connections, connection control (path selection), network-dependent error monitoring, end system coupling.Layer 4: Transport layer:        Layer 4 sets up, controls and terminates the transport connection leading from one end system to the other. It comprises the following functions: adaptation to different network properties, end-to-end error control, address translation (e.g. name to telephone number), data segmentation.Layer 5: Communication control (session layer):        Layer 5 serves to start a communication, ensure its proper execution and its termination. It comprises the functions of connecting and maintaining logical connections, connection identification and dialogue control.Layer 6: Presentation layer:        The presentation protocol decides how the information items are to be exchanged and presented in a common language. Layer 6 comprises the functions of syntax selection in line with the application, format adaptation, code and alphabet conversion.Layer 7: Application layer:        In layer 7 the system and application control actions are carried out. It comprises the functions of identification of communication partners, authorisation check for communication, access to communication, selection of transfer quality and transfer parameters.        
One layer in the reference model is represented by a so-called entity. The entity provides the functions assigned to it in the layer model. This way it provides a service for the next higher layer. One example is the service of layer 2 which securely transfers layer 3 information for layer 3. This service provisioning is not only used by the next higher layer but by all layers situated above it. A higher layer thus uses the sum of all layers situated below it. The entities of the layers communicate only with the respective neighbouring layers (towards the top and the bottom).
FIG. 3 shows the communication between the layers as an example for any three layers 20a, 20b, 20c. In order to be able to provide the service of a particular layer, the relevant entity communicates with the entity of the same layer in the peer system, its peer entity. The communication with the peer entity takes place through the exchange of messages. The prescribed exchange of such messages is called protocol. Each layer has such a layer protocol to be able to provide to the higher-level layers the service assigned to it in the reference model. The higher layers see nothing of this protocol. They are only presented with the result of the service provisioning. In this way, when the information items run through the individual layers, protocol data units are added by each layer to the actual information items to be transferred. On the other hand, when the information items run through from bottom to top, protocol data units are removed until, finally, the actual information items to be transferred remain. The layer protocol determines the outward behaviour of a system and is therefore generally standardized.
Each entity requests the assigned functions from the respective lower layers. All higher layers in the reference model are represented by a service access point located directly under it. The communication between the layers takes place via so-called service elements (communication primitives, or simply primitives—elementary messages that cannot be subdivided). The communication within a system is not normally standardized.
Open communication systems are checked using test apparatuses such as protocol testers. Said protocol testers have the task of testing the service quality of telecommunication connections by, on the one hand, monitoring communications actually taking place and making such communications the basis of an analysis, and on the other hand, by stimulating participants in the communication with test communications. In the first case, a distinction is made between two possibilities: firstly, the relevant information transmitted through the communication line is duplicated, see FIG. 4b, the duplicate being transferred to protocol tester 22. In a second possibility, see FIG. 4a, the protocol tester 22 is arranged serially in the communication path between two end systems 12a and 12b, i.e., the whole communication takes place across the protocol tester 22. With the method known from the prior art, see FIG. 4a and FIG. 4b, outputting of the communication to the protocol tester 22 takes place at a transit system 16.
FIG. 5 shows the interconnection of two end systems and one test apparatus 22 for the methods shown in FIGS. 4a and 4b and known from the prior art. In this drawing plane, the transit system 16 is situated behind the test apparatus 22 and is therefore not visible. With this prior art, the problem is that test apparatus 22 also has to have a physical interface in order to make available the information transported via the transmission medium, as indicated by lines 26a to g in FIG. 5. In case that only information of a low layer, such as layer 2 or layer 3, is important for an operator who accesses the test apparatus 22 via a port 23, further processing of the information by higher-level layers may be dispensed with, i.e., providing lines 26a, 26b is sufficient. Accordingly, only the processing of the information for the lower-level layers has to be realized in the test apparatus 22. What cannot be avoided, however, is the need for an interface for processing the information for layer 2, which means that the layer 1 functions have to be implemented in said interface. Particularly for high-speed connections, realizing a physical interface requires great technical effort and is thus costly.
What is desired is to avoid the disadvantages of the prior art and to provide a circuit arrangement and/or a method that allows a test apparatus to test a communication system with as little effort as possible.